IMAGING AND SPATIO-TEMPORAL ANALYSIS OF TURBULENT MIXING OF HYDROTHERMAL WATER DISCHARGING INTO A RIVER (BREITENBUSH HOT SPRINGS, OREGON)

We investigated the thermal plume created by a pipe discharging hot spring water (~60 °C) into a small stream (~10 °C) at Breitenbush Hot Springs, Oregon. High-frequency (16 Hz), high-resolution (1-2 mm pixels) thermal infrared images of hot spring jets show the effects of jet entry conditions on spatial and temporal scales of mixing. Images of thermal plumes showing eddy cascades through space and time are analyzed via correlation analysis. The 3-inch diameter discharge pipe was positioned such that the jet is at the surface, partially submerged, or at the bottom of the 15-cm deep stream. The three jet entry conditions are hereafter referred to as “shallow”, “middle”, and “deep”. In the shallow and middle positions, the jet is apparent on the stream surface as a hot region extending downstream from the pipe. Turbulent mixing between the jet and the stream occurs along the jet margins, such that the discharge plume broadens and cools downstream. In the deep position, the jet reaches the surface as a broad plume ~7.5 cm downstream from the pipe; the highest measured temperatures are not directly above the pipe mouth, but displaced downstream. The average downstream distance at which 50% mixing occurs changes from ~40 cm in shallow and middle positions to ~ 30 cm in the deep position. The width of the mixing plume also changes from ~7.5 cm in shallow and middle positions to ~10 cm in the deep position. Streamwise spatial autocorrelation analysis of the temperature field under shallow and middle entry conditions show correlation length scales of ~30 cm for a transect along the center of the jet; the correlation length scale abruptly reduces to <10 cm on either side of the jet. Under deep conditions, the streamwise correlation length scale is ~20 cm along the plume center and ~10 cm on either side of the plume. Temporal autocorrelation analysis of the temperature fields shows quasi-periodicity for all three pipe positions and decrease in frequency with distance from the pipe (shallow and middle) or center of the upwelling plume (deep). This coincides with increase in eddy size. The use of thermal imaging technology allowed us to quantitatively analyze mixing of thermal plumes at a level of detail rarely achieved.